Recently, digital broadcasting for transmitting and receiving image data as digital data has been noted. An advantage of digital broadcasting is that a greater quantity of program data (hereinafter referred to as programs) can be transmitted on the same transmission line. This is considerably owing to the fact that image data can be compressed and transmitted. As a method for compressing image data, a bidirectionally predictive coding system employed in the MPEG (Moving Picture Experts Group) standard is often used. In this bidirectionally predictive coding system, three types of coding is carried out, that is, intra-frame coding, inter-frame forward predictive coding and bidirectionally predictive coding. Pictures of these coding types are called I-picture (intra coded picture), P-picture (predictive coded picture), and B-picture (bidirectionally predictive coded picture), respectively.
In compression coding of pictures in digital broadcasting, it is necessary to maintain high picture quality while restraining the data quantity (bit quantity) after compression coding to not more than the transmission capacity of the transmission line.
As a method for transmitting a greater number of programs to a transmission line of a predetermined transmission capacity, a technique of "statistical multiplexing" is known. The statistical multiplexing technique is a technique for dynamically changing the transmission rate of each program so as to transmit a greater number of programs. In this statistical multiplexing, transmission of a greater number of programs is made possible by reducing the transmission rate of a program such that deterioration in picture quality is imperceptible even when the transmission rate is reduced.
Referring to FIGS. 1 and 2, statistical multiplexing will be described further in detail. FIG. 1 shows an exemplary allocation bit rate for each program in the conventional case where multiplexing is carried out at a fixed rate. The vertical axis represents the allocation bit rate for each program and the horizontal axis represents the time. As shown in FIG. 1, the allocation bit rate of each program to be multiplexed such as weather forecast, news or drama remains constant from the bit rate allocated as an initial value, and does not vary with the lapse of time. The bit rate allocated to each program as the initial value is allocated so that deterioration in picture quality of a part (time) where deterioration in picture quality of each program is most perceptible falls within an allowable range. Therefore, a bit rate which is more than necessary is not allocated to parts except for the part where deterioration in picture quality is perceptible.
FIG. 2 shows an exemplary allocation bit rate for each program in the case where multiplexing is carried out by dynamically changing the allocation bit rate for each program using the statistical multiplexing technique. The vertical axis represents the allocation bit rate for each program and the horizontal axis represents the time. Statistical multiplexing utilizes the fact that a part (time) where deterioration in picture quality of each program is perceptible is rarely generated in the same time zone. That is, when deterioration in picture quality is perceptible in one program, deterioration in picture quality of other programs is often imperceptible even though the bit rate is lowered. Therefore, it is possible to lower the bit rate of the other programs and allocate a higher bit rate to the program having perceptible deterioration in picture quality. In the example of FIG. 2, at a time point t1, the image of drama (1) has perceptible deterioration in picture quality but the images of news and drama (2) have imperceptible deterioration in picture quality. Therefore, the bit rate of the images of new and drama (2) is lowered and a higher bit rate is accordingly allocated to the image of drama (1). By thus using the statistical multiplexing technique, a greater number of programs can be transmitted.
However, there arises a problem of phase of GOP (group of pictures). Normally, an encoder encodes images while carrying out rate control on the GOP basis, as represented by TM5 (Test Model Editing Comnmittee: "Test Model 5"; ISO/IEC JTC/SC292/WG11/N400 (April1993)). That is, the bit rate is not changed at a halfway point of the GOP. If the phases of GOP of the respective programs are matched (which rarely occurs in general), there is no problem. However, if the phases of GOP have discrepancy (which generally occurs in most cases), since the bit rate of the program is changed only at the leading end of the GOP, the bit rates of all the programs cannot be changed at a time. Therefore, a dead space (a portion where the bit rate is not allocated to any program) is generated, as shown in FIG. 3, and all the transmission capacity cannot be effectively used.
Thus, it may also be considered to employ a technique for encoding images while determining the target bit quantity for each frame unit instead of carrying out rate control on the GOP basis. However, in the case of digital broadcasting, it is necessary to strictly control the quantity of generated bits so as not to generate overflow or underflow of a VBV (video buffering verifier) buffer of a receiver (decoder). If the target bit quantity is determined for each frame unit, it is difficult to control the quantity of generated bits so as not to generate overflow or under of the VBV buffer.